(a) Field of the Invention
This invention relates to perforated polyolefin films which films are mainly used in the fields of sanitary, food packaging and industrial materials.
(b) Description of the Prior Art
Since perforated films are permeable to gases and liquids, the films have been recently investigated for use in sanitary materials and other broad areas by utilizing their permeability to gases and liquids.
As to preparation methods of the perforated films used in these areas, methods disclosed in Japanese Laid-Open Patent No. 91486/1978 and Japanese Patent Publication No. 36130/1982 are known. In these methods, films are processed by heating with hot air on the surface of bored drums and then vacuuming the inside of the drums to perforate the films. Although these methods are reliable and effective, equipment is complex and expensive, and processing at a low speed is also inevitable. Accordingly, these methods have a disadvantage of generally leading to high cost.
An improvement of the above methods has been proposed in U.S. Pat. Nos. 4,272,473 and 4,319,868 (Japanese Patent Publication No. 25577/1983). In this method, a film is pinched with pressure on an embossing roll having a large number of protruded portions, followed by pressing with a hot metal roll to perforate the film by the protruded portions. Then the perforated film is pinched again with pressure and cooled. This method is better than the above-mentioned methods but has a low processing speed of about 9.4 m/min., thereby causing high cost. It is also difficult in this method to operate for a long period of time because the perforating effectiveness is reduced by abrasion of the convex surface of the metal rolls.
In addition, a method for opening the recessed portion of the film by orienting a previously embossed film is disclosed in Japanese Patent Publication No. 27097/1983. (See U.S. Pat. Nos. 4,101,358 and 4,144,368.) The method is poor in stretching workability and also has a disadvantage of resulting in enlarged pores.